This relates to transient energy systems for supplying power to a load substantially instantaneously on demand. More particularly, this relates to flywheel energy storage systems constructed with an induction motor-generator.
There are many different types of transient systems, including, for example, batteries, ultra-capacitors, and flywheel energy storage systems, all of which are capable of providing power substantially immediately on demand. Batteries have poor reliability, are bulky (thus requiring substantial storage space), are expensive to maintain, and contain environmentally damaging chemicals. Ultra-capacitors are expensive and their development has yet to achieve economies of scale which would permit their widespread use. An example of a flywheel energy storage system includes the relatively high mass flywheel type of system. In this type of flywheel energy system, the flywheel is contained in an airtight container under vacuum to reduce drag on the flywheel's rotation. A disadvantage of such flywheel systems is that they become relatively large (thereby occupying substantial floor space), heavy and costly, in applications where longer runtimes are desired.
Transient systems may be used in combination with a “long-term” backup power generation system (which systems may not be capable of supplying power substantially immediately on demand) to form an uninterruptible power supply (UPS) system. A UPS system may be operative to ensure that there is no interruption in the supply of power to a critical load in the event of an interruption in the supply of power from the primary power source (e.g., a utility power failure). A critical load may be a load requiring a continuous uninterrupted supply of power such as, for example, telecommunications systems, data centers, and other power sensitive loads. The transient energy storage system may bridge the supply of power substantially immediately after interruption in the supply of power from a primary power source (e.g., utility power) at least until a “long-term” backup power generation system (e.g., a compressed air storage system) is “activated” and able to supply power to the critical load.
Because interruptions in the supply of power from a primary power source are relatively rare, the need for transient systems to supply power on demand may occur only a few times during the life of such systems. However, when backup power is needed, the demands on the transient systems are high. The following considerations may be taken into account for transient system, and in particular, flywheel energy storage systems:
(1) the operational life of the flywheel system;
(2) the power consumed during a standby mode of operation (or motoring mode of operation);
(3) the quantity of power generated during a generation mode of operation;
(4) the response time in which real power is generated;
(5) the heat losses generated during standby mode;
(6) the ability to cool components during standby and/or generation modes;
(7) the wear and tear on components such as bearings;
(8) the audible noise;
(9) the range of voltage on a DC bus;
(10) the flywheel rotational speed;
(11) the stresses on the flywheel;
(12) the ability to control real power generated by the flywheel system;
(13) the control circuitry current limits; and
(14) the magnetic saturation limits.
There are several known configurations of flywheel energy storage systems (see, for example, U.S. Pat. No. 5,731,645). These systems maximum energy storage and require vacuum sealed enclosures to minimize flywheel windage losses. Some of the components within these systems can be highly customized and require manufacturing methods that are not widely used in industry thereby increasing cost.
Another type of system may be used to generate power, but is not conducive to transient power generation, includes conventional induction motor/generators that are driven by a prime mover (e.g., engine, wind, water, etc.) to generate power. Such induction motor/generators are designed and used for continuous duty operation. As such, the induction motor/generator is constructed to handle thermal and electrical limits for its power rating, resulting in larger sizing (and associated higher cost) and limited ability to rapidly transition from a motor mode to generator mode.
What is needed is an improved flywheel energy storage system constructed with a low-cost induction motor/generator that is optimized for transient power generation and methods for use of the same.